This invention relates to a polymer piezoelectric film having high piezoelectric performance even in a high frequency region and also endowed with excellent element manufacturability and to a process for producing the same, particularly to a polymer piezoelectric film suitable for ultrasonic Etransducer and to a process for producing the same.
As a polymeric piezoelectric material having high piezoelectric performance, U.S. Pat. No. 3931446 for example, discloses polyvinylidene fluoride. Also reported are vinylidene fluoride copolymers in U.S. Pat. No. 4204135, Japanese Pat. (Kohkai) Nos. 56-111281 and 58-60585. Moreover, these vinylidene fluoride resins are reported to have high piezoelectric performance even in a high frequency region, as disclosed in U.S. Pat. No. 3798473. These polymeric piezoelectric materials are generally formed into piezoelectric films by forming them into films according to rolling or casting, then applying heat treatment on the film surface in order to enhance the electro-mechanical coupling factor k.sub.t (hereinafter frequently referred to merely as "k.sub.t ") in the direction perpendicular thereto and applying an electric field in the direction perpendicular to the film to effect poling treatment. Among a series of these vinylidene fluoride resins, polyvinylidene fluoride has a k.sub.t value of about 0.2 and is also excellent in formability, thus being expected to be the most promising polymeric element material for ultrasonic transducers. However, further improvement of the electro-mechanical coupling factor k.sub.t is desired.
On the other hand, the vinylidene fluoride copolymers disclosed in the above-mentioned Patent Publications, particularly the copolymer of vinylidene fluoride and trifluoroethylene, having a composition of about 75 mol. % of vinylidene fluoride and about 25 mol. % of trifluoroethylene, either in the form of a rolled or a casted film, has a k.sub.t value surpassing that of polyvinylidene fluoride. However, the film product of this copolymer is susceptible to generation of cracks, when processed to have a concave plane for the purpose of enhancing transmitting and receiving sensitivities of an ultrasonic transducer, and therefore production yield of elements is very poor.
Cracks are liable to be generated because crystallinity is extremely increased when a vinylidene fluoride-trifluoroethylene copolymer is heat-treated for enhancing k.sub.t, as compared with a polyvinylidene fluoride. In this connection, when heat treatment is suppressed by shortening the time therefor to the extent that the production yield of element is not worsen, the k.sub.t value is merely slightly higher than that of polyvinylidene fluoride, whereby formation of the copolymer is of little significance.